Ships, cars, countries — people ascribe gender to all these things, even though none of them have genitals. But a new study reveals an even weirder mental habit — people apparently think of odd numbers as male, while even ones are female.

According to Scientific American (via LiveScience), psychologists found the odd-is-male effect in a variety of experiments. In one test, they asked participants to describe unfamiliar names as male or female. The subjects were more likely to designate a name as male when it was paired with the number 1; when paired with the number 2, names seemed more female. They performed a similar pairing test with photos of babies, and got the same result: babies numbered 1 were more likely to be judged male. Finally, they asked subjects to simply rate the masculinity and femininity of different numbers — they rated odds as more masculine, evens as more feminine. This result was true for both American and Indian study populations, suggesting that odd numbers are dudes in at least two different cultures.

Scientific American's Daisy Grewal writes,

Why would odd numbers, across cultures, be associated with masculinity? While more research is needed for a solid answer, it may have something to do with gender stereotypes. [Study authors] Wilkie and Bodenhausen plan to explore this hypothesis in future studies by seeing whether people associate odd numbers with more stereotypically masculine qualities, such as dominance and independence.

One possibility, at least in the first two experiments, is that people associate masculinity with the number 1 because of unconscious beliefs that men are better than women or belong in first place. However, that doesn't explain why, say, 5 is also perceived as masculine. Grewal notes that "people see furniture items, such as tables and trash cans, as more feminine when they feature rounded, rather than sharp, edges" — maybe odd numbers feel sharper or harsher than evens. Or maybe odd numbers seem a little phallic — after all, if you divide them in half, there's still something dangling in the middle. Whatever the case, the study is a fascinating illustration of our desire to slap gender on things that obviously lack lady- or man-parts. Now it's time to figure out why we feel the need to do this.